I found a petal from the rose bush outside of my studio, and decided to fashion one with a similar shape out of a triangle using the Pazzles InVue software. You can find the cutting file for the resulting 3DRose in the Pazzles Craft Room Library. These files came in handy for creating paper bouquets for my son's wedding.
Here's how I assembled the roses. The first layer formed the flower bud when the sides of one petal were hot glued to an opposite petal. The remaining 3 petals were hot glued about 1/3 of the way up from the base of each petal, and adhered to the forming bud. The back center of the next layer was hot glued to the first, with the petals positioned alternately to those of the first. A thin line of hot glue was applied to the base of each petal, which was folded up to adhere to the forming bud. The corners of each petal were curled away from the center of the bud using the small end of a small paint brush. This process was repeated with all of the layers except the last one. The last layer was simply glued in the center back, with no glue required at the base of the petals.